How to apply operation every nth element

Solution 1:

I believe this is what you are looking for:

public static void main(String[] args) {
    String string = "12345";
    String result = IntStream.range(0, string.length())
            .mapToObj(index -> incrementCharAtEvenIndex(string, index, 3))
            .collect(Collectors.joining());

    System.out.println(result);
}

static String incrementCharAtEvenIndex(String string, int index, int increment) {
    int number = Character.getNumericValue(string.charAt(index));
    if ((index + 1) % 2 == 0) {
        number = number + increment;
    }
    return String.valueOf(number);
}

Unfortunately, Stream API is not built to work with the indices directly, so you have to use IntStream to simulate for-loop and a custom method to increment a numeric character if it appears at the even index (remember it starts at 0).

Solution 2:

You can use the range operator to add a index to your stream:

String myString = "12345";
int n = 2;
        
IntStream
  .range(0, myString.length())
  .mapToObj(i -> Map.entry(i, Character.getNumericValue(myString.charAt(i))))
  .mapToInt(pair -> (pair.getKey() + 1) % n == 0 ? pair.getValue() + 3 : pair.getValue())
  .forEach(System.out::print);